style(9) rules for nested includes

[mdf at FreeBSD.org]

On Thu, Mar 10, 2011 at 11:46 AM, John Baldwin <jhb at freebsd.org> wrote:
> On Thursday, March 10, 2011 12:17:28 pm mdf at freebsd.org wrote:
>> I recall a recent discussion/PR about nested includes in the context
>> of <sys/linker_set.h> and <sys/queue.h> being a few of the only ones
>> allowed.  However, I don't see anything in style(9) about this.
>
> bde@ is probably the most authoritative.  My understanding is that the only
> nested includes allowed in sys/sys/*.h are the two listed above and any header
> that starts with an underscore (sys/_mutex.h, etc.).  The underscore variants
> were added to allow nested includes when absolutely necessary, but those
> includes are the bare minimum required to define structures, etc.
>
>> Now we come to the reason I ask.  I'm working on a patch to change the
>> static sysctl code to use the standard SYSININT/SYSUNINIT code rather
>> than have special treatment in kern_linker.c, but to do this I need to
>> either change quite a few places that include <sys/sysctl.h>, or
>> include <sys/kernel.h> instead of <sys/linker_set.h> in sysctl.h, as
>> the SI_SUB_SYSCTLS value isn't visible otherwise.
>
> Hmm, what is the reason to use SYSINIT's instead of a dedicated linker set?

Mostly for consistency.  The DB_COMMAND linker set was changed to use
SYSINITs for version 8, and AFIAK SYSCTL is the only global kernel
thing using a separate linker set.

There's also a minor bug in initialization ordering where a static
SYSCTL_PROC could use a lock initialized by SX_SYSINIT or MTX_SYSINIT,
but at runtime module load the sysctl is exposed before the
SI_SUB_LOCK stage has run, so in theory someone doing sysctl -a would
crash the kernel on an attempt to lock an uninitialized mtx/sx.  We
saw this happen once at Isilon.

Thanks,
matthew